There is a type of insulated gate bipolar transistor (IGBT) with a trench gate structure that includes dummy trenches (which do not constitute part of the channels) to reduce the current in short circuit mode and prevent electrical breakdown (see, e.g., JP-A-2002-16252).
FIG. 4 is a top view of a conventional insulated gate transistor. Referring to the figure, dummy trenches 17 are formed on both sides of trenches 14, and n-type source layers 21 are selectively formed in the surface of a p-type base layer 13 and in contact with the sidewalls of the trenches 14. Further, p+ type contact layers 22 are formed in the surface of the base layers 13 and between the trenches 14 and the dummy trenches 17.
Thus in conventional insulated gate transistors, the source layers 21 are formed between the contact layers 22 and the trenches 14, and these source layers 21 act as source ballast resistances 27 (see FIG. 4). It has been found, however, that the values of the ballast resistances 27 vary with variations in the patterning accuracy of the source and trench forming processes, which results in variations in current in short circuit mode and which may result in reduced resistance to electrical breakdown.